Image forming apparatus

ABSTRACT

An image forming apparatus includes: a power source portion; a fixing portion; and a controller. The controller sets a waveform of a current to be passed through each of first and second heat-generating-elements of the fixing portion in one-control-period so that, in an equiphase half wave in one control period, the current passes through one heat-generating-element from a halfway point of the half wave and the current passes through or does not pass through the other heat-generating-element throughout a half wave period. The controller sets a current supply starting timing of the current passing through the one heat-generating-element, at timing when a current passing toward the power source portion stops.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an image forming apparatus using anelectrophotographic process.

In recent years, also in the image forming apparatus using theelectrophotographic process, there is a tendency that electric powerconsumed by a fixing device is increased with speed-up anddiversification of options, and thus capacity of a low-voltage powersource portion is increased. In such a situation, in order to lowermaximum current consumption by increasing a power factor of thelow-voltage power source portion, e.g., as described in JapaneseLaid-Open Patent Application (JP-A) 2006-304534, there is an increasingcase where a switching power source in which a power factor improvingcircuit such as a step-up active filler is mounted has been used.

However, the power factor improving circuit has a complicated circuitstructure and thus leads to an increase in cost of the low-voltage powersource portion, and due to an increase in circuit, a large space isneeded for providing the low-voltage power source portion. For thisreason, it has been desired that the power factor is improved.

SUMMARY OF THE INVENTION

The present invention has been accomplished in the above-describedsituation. A principal object of the present invention is to provide animage forming apparatus capable of improving a power factor whilesuppressing a cost.

According to an aspect of the present invention, there is provided animage forming apparatus comprising: a power source portion forconverting an AC voltage of a commercial power source into a DC voltage;a fixing portion for heating and fixing an image, formed on a recordingmaterial, on the recording material, wherein the fixing portion includesa first heat generating element which generates heat by electric powersupplied from the commercial power source and a second heat generatingelement which is controlled independently of the first heat generatingelement and which generates the heat by the electric power supplied fromthe commercial power source; and a controller for controlling theelectric power supplied to the first and second heat generatingelements, wherein when a plurality of periods of an AC waveform of thecommercial power source constitute one control period, the controllersets a waveform of a current to be passed through each of the first andsecond heat generating elements in the one control period so that totalelectric power supplied to the first and second heat generating elementsin the one control period is dependent on a temperature of the fixingportion, wherein the controller sets the waveform of the current to bepassed through each of the first and second heat generating elements sothat, in an equiphase half wave in at least a part of the one controlperiod, the current passes through one of the first and second heatgenerating elements from a halfway point of the half wave and thecurrent passes through or does not pass through the other heatgenerating element throughout a period of the half wave, and wherein thecontroller sets a current supply starting timing of the current passingthrough the one of the first and second heat generating elements fromthe halfway point of the half wave, at timing when a current passingtoward the power source portion stops.

According to another aspect of the present invention, there is providedan image forming apparatus comprising: a fixing portion for heating andfixing an image, formed on a recording material, on the recordingmaterial, wherein the fixing portion includes a first heat generatingelement which generates heat by electric power supplied from thecommercial power source and a second heat generating element which iscontrolled independently of the first heat generating element and whichgenerates the heat by the electric power supplied from the commercialpower source; and a controller for controlling the electric powersupplied to the first and second heat generating elements; wherein thecontroller switches a rule of a waveform of an AC current to be passedthrough each of the first and second heat generating elements dependingon a duty ratio of total electric power supplied to the first and secondheat generating elements.

According to a further aspect of the present invention, there isprovided an image forming apparatus comprising: a power source portionfor converting an AC voltage of a commercial power source into a DCvoltage; a fixing portion for heating and fixing an image, formed on arecording material, on the recording material, wherein the fixingportion includes a first heat generating element which generates heat byelectric power supplied from the commercial power source and a secondheat generating element which is controlled independently of the firstheat generating element and which generates the heat by the electricpower supplied from the commercial power source;

a current detecting portion for detecting a resultant current passingthrough the power source portion and the first and second heatgenerating elements; and a controller for controlling the electric powersupplied to the first and second heat generating elements, wherein thecontroller sets a length of the one control period depending on adetected current of the current detecting portion.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are schematic views showing structures of an imageforming apparatus, a fixing device, a heater and the heater,respectively.

In FIGS. 2, (a) and (b) are diagrams of a heater driving circuit and alow-voltage power source circuit, respectively, in Embodiment 1.

In FIG. 3, (a) to (e) are waveform charts each showing a suppliedelectric power pattern to the heater in Embodiment 1.

In FIG. 4, (a) to (d) are waveform charts each showing a power sourcecurrent, a resultant current of the heater and an inlet current inEmbodiment 1.

In FIGS. 5, (a) and (b) are graphs showing relationships, in Embodiment1 and Comparison Example, respectively, between supplied electric powerand a power factor.

In FIG. 6, (a) to (d) are tables showing supplied electric powerpatterns to heaters in Embodiment 1 and Comparison Example.

FIG. 7 is a flowchart showing a process of setting the supplied electricpower pattern in Embodiment 1.

In FIGS. 8, (a) and (b) are diagrams of a heater during circuit and alow-voltage power source circuit, respectively, in Embodiment 2.

In FIG. 9, (a) to (h) are waveform diagrams for illustrating anoperation of an inlet current detecting circuit in Embodiment 2.

FIG. 10 is a flowchart showing a process of setting a supplied electricpower pattern in Embodiment 2.

FIG. 11 is a graph showing a relationship between a supplied electricpower to a heater and a square value of an effective current inEmbodiment 3.

In FIG. 12, (a) to (c) are graphs each showing an inlet current inEmbodiment 3.

FIG. 13 is a flowchart showing a detecting process of timing inEmbodiment 3.

FIGS. 14 and 15 are flowcharts each showing a discriminating process asto whether or not a change in timing is needed in Embodiment 4.

DESCRIPTION OF THE EMBODIMENTS

With reference to the drawings, embodiments for carrying out the presentinvention will be specifically described below. However, dimensions,materials, shapes and relative arrangement of constituent elementsdescribed in the following embodiments should be appropriately changeddepending on structure and various conditions of devices (apparatuses)to which the present invention is to be applied. That is, the scope ofthe present invention is not intended to be limited to the followingembodiments.

Embodiment 1 General Structure of Image Forming Apparatus

FIG. 1A is a schematic illustration of a color image forming apparatusof a tandem type using an electrophotographic process in Embodiment 1.The image forming apparatus in this embodiment is constituted so that afull-color image can be outputted by superposing toner images of fourcolors of yellow (Y), magenta (M), cyan (C) and black (K). In thefollowing description, in the case where there is a need to designate apredetermined color, members or portions are represented by adding Y, M,C, K to reference numerals or symbols, and in the case where there is noneed to designate the predetermined color, the members are presented bythe reference numerals without adding Y, M, C, K. In the image formingapparatus in this embodiment, in order to form the respective colortoner images, laser scanners 11 and cartridges 12 are provided. Thecartridges 12 one constituted by photosensitive drums 13 as imagebearing members rotatable in arrow (clockwise direction) directions inFIG. 1A and developing devices including developing rollers 16. Thecartridges 12 include photosensitive drum cleaners 14 provided incontact with the photosensitive drums 13, charging rollers 15 and thedeveloping rollers 16. Further, the respective photosensitive drums areprovided in contact with an intermediary transfer belt 19, and primarytransfer rollers 18 are provided opposed to the photosensitive drums 13via the intermediary transfer belt 19.

In the neighborhood of a cassette 22 for accommodating a sheet 21 as arecording material (medium), a sheet presence/absence detecting sensor24 for detecting the presence or absence of the sheet 21 in the cassette22 is provided. Further, in a feeding path of the sheet 21, a sheetfeeding roller 25, separation rollers 26 a and 26 b and a registerroller pair 27 are provided, and in the neighborhood of the registerroller pair 27 in a downstream side with respect to a sheet feedingdirection, a register sensor 28 is provided. In a further downstreamside of the feeding path with respect to the sheet feeding direction, asecondary transfer roller 29 is provided in contact with theintermediary transfer belt 19, and a fixing device 30 is provideddownstream of the secondary transfer roller 29.

A controller 31 as a control portion of the image forming apparatus isconstituted by a CPU (central processing unit) 32 including ROM 32 a,RAM 32 b, a timer 31 and the like, and by various input/output controlcircuits (not shown) and the like.

Next, the electrophotographic process will be briefly described.Surfaces of the photosensitive drums 13 are electrically chargeduniformly by the charging rollers 15. Then, the surfaces of thephotosensitive drums 13 are irradiated, by the laser scanners 11, withlaser light modulated depending on image data. Electric charges at aportion where the photosensitive drum is irradiated with the laser lightare removed, so that electrostatic latent images are formed on thesurfaces of the photosensitive drums 13. As a result, the respectivecolor toner images are formed on the surfaces of the photosensitivedrums 13.

The toner images formed on the photosensitive drums 13 are attractedtoward the intermediary transfer belt 19 at nips between theintermediary transfer belt 19 and the respective photosensitive drums 13by the primary transfer rollers 18 to which a primary transfer voltageis applied. Then, the respective color toner images are successivelytransferred onto the intermediary transfer belt 19, so that a full-colorimage is finally formed on the intermediary transfer belt 19.

On the other hand, the sheet 21 in the cassette 22 is fed by the sheetfeeding roller 25, and then by the separation rollers 26 a and 26 b, thesheet 21 is passed through the register roller pair 27 one by one andthen is conveyed to the secondary transfer roller 29. At a nip betweenthe intermediary transfer belt 19 and the secondary transfer roller 29disposed downstream of the register roller pair 28, the toner image istransferred from the intermediary transfer belt 19 onto the sheet 21.Finally, the toner image on the sheet 21 is heat-fixed by the fixingdevice 30 as a heating portion, and the sheet 21 is discharged to anoutside of the image forming apparatus.

[Fixing Device]

FIG. 1B is a schematic sectional view of the fixing device 30 in thisembodiment. The fixing device 30 is a heating device (apparatus) of apressing roller drive type and of a film heating type using, e.g., anendless film (cylindrical film), and generally has the followingstructure. The fixing device 30 includes a heater 100 as a heatingmeans, and a heater holder 101 on which the heater 100 is fixed andheld. The fixing device 30 further includes a cylindrical thinheat-resistant film (fixing film) 102 externally fitted loosely aroundthe heater holder 101 on which the heater 100 is mounted. The fixingdevice 30 includes a pressing roller 103 press-contacted to the fixingfilm 102 toward the heater 100 to form a fixing nip N, and a protectiveelement (thermo-switch) 104 provided on the surface of the heater sothat a heat-sensitive surface thereof contacts the surface of the heater100.

The pressing roller 103 is rotationally driven, by an unshown drivingmotor or the like as a driving means, in the counterclockwise directionindicated by an arrow in FIG. 1B at a predetermined peripheral speed. Bya press-contact frictional force at the fixing nip N between the outersurface of the pressing roller 103 and the fixing film 102, a rotationalforce of the pressing roller 103 acts on the cylindrical fixing film102, so that the fixing 102 is placed in a state in which the fixingfilm 102 is rotated by rotation of the pressing roller 103. The fixingfilm 102 is rotated around the heater holder 101 in the clockwisedirection indicated by an arrow in FIG. 1B while being slid in closecontact with a downward surface of the heater 100 at an inner surfacethereof.

A temperature of the heater 100 is increased up to a predeterminedtemperature (control target temperature) by supplying electric power tothe heater 100, and then temperature control is effected for maintainingthe heater temperature at the predetermined temperature. In thistemperature-controlled state, the sheet 21 on which an unfixed tonerimage T is carried is fed in a sheet feeding direction (leftwarddirection in (b) of FIG. 1). In the fixing nip N, a toner image-carryingsurface of the sheet 21 hermetically contacts the outer surface of thefixing film 102 and then the sheet 21 is nipped and fed together withthe fixing film 102 through the fixing nip N. In this nip-feedingprocess of the sheet 21, heat of the heater 100 is imparted to the sheet21 via the fixing film 102, so that the unfixed toner image T on thesheet 21 is heated and pressed to be melt-fixed. The sheet 21 passingthrough the fixing nip N is curvature-separated from the fixing film103.

FIG. 1C is an enlarged sectional view of the heater 100. The heater 100is a ceramic heater of a back-surface heating type. The ceramic heater100 is constituted by an insulating substrate of a ceramic material suchas SiC, AlN or Al₂O₃, a first heat generating element 111 and a secondheat generating element 112 which are formed on the insulating substrate110, and a protective layer 113, such as glass, for protecting the twoheat generating elements 111 and 112. Further, a glass layer forimproving a sliding property with the fixing film 102 is formed on asurface, of the insulating substrate 110, opposite from the surfacewhere the heat generating elements 111 and 112 is printed. The sheet 21is fed from right to left in FIG. 10, and the right side is an upstreamside and the left side is a downstream side with respect to the feedingdirection of the sheet 21.

FIG. 1D is a schematic plan view of the heater 100. The first heatgenerating element 111 includes a heat generating portion, electrodes111 c and 111 d, and electroconductive portions 111 b for connecting theheat generating portion 111 a with the electrodes 111 c and 111 d, andelectric power is supplied to the heat generating portion 111 a via theelectrodes 111 c and 111 d, so that the heat generating portion 111 agenerates heat. Similarly, the second heat generating element 112includes two heat generating portions 112 a, electrodes 112 c and 112 d,and an electroconductive portion 112 b for connecting the two heatgenerating portions 112 a, and the electric power is supplied to theheat generating portions 112 a, so that the heat generating portions 112a generate the heat. Further, the electric power supply is effected viaconnectors 114 and 115, for electric power supply, each indicated by achain double-dashed line in FIG. 1D.

A chain line shown in FIG. 1D, is a center feeding reference line in thecase where the sheet 21 is fed so that a central portion thereof withrespect to a direction perpendicular to the feeding direction of thesheet 21 is aligned with a central portion of the feeding path withrespect to the direction perpendicular to the feeding direction. Aregion L1 is a sheet passing width region of a passable maximum-sizedpaper, and a region L2 is a sheet passing width region of a passableminimum-sized paper. The sheet passing width is a length of the sheet 21with respect to the direction perpendicular to the feeding direction ofthe sheet 21. The heat generating element 111 is principally used forheating the central portion of the sheet 21, and the heat generatingelement 112 is principally used for heating end portions of the sheet21.

[Heater Driving Circuit]

In FIG. 2, (a) is a circuit diagram for illustrating a heater drivingcircuit in this embodiment. A commercial power source (AC power source)50 to which the image forming apparatus is to be connected supplies ACelectric power to the image forming apparatus via an inlet 51. Theheater driving circuit is constituted by a primary-side portion directlyconnected to the commercial power source 50 and a secondary-side portionconnected to the heat generating element 50 in a non-contact manner. Theelectric power inputted from the commercial power source 50 is suppliedto the heat generating elements 111 and 112 via an AC filter 52 to causethe heat generating elements 111 and 112 to generate heat. The electricpower of the commercial power source is also inputted into the powersource portion 53 via the AC filter 52, so that the power source portion53 outputs a predetermined voltage to a secondary-side load. The powersource portion 53 converts an AC voltage of the commercial power sourceinto a DC voltage and outputs, the DC voltage. For example, the powersource portion outputs a voltage of 3 V for driving the CPU 32 or thelike and a voltage of 24 V for driving the motor or the like. Further,the CPU 32 is also used in the heater drive control and the like, and isconstituted by input and output ports, the ROM 32 a, the RAM 32 b, andthe like.

In the image forming apparatus, in the primary side of the electricpower supplying circuit, a constitution in which the heat generatingelements 111 and 112 of the fixing device 30 and a power source unit(power source portion) 53 for supplying the electric power to thesecondary side are directly connected with the commercial power sourceto be supplied with the electric power is employed. Further, in thesecondary side of the electric power supplying circuit, a constitutionin which the motor and units, to be operated during image formation,such as the motor for rotating the photosensitive drums 13 and theintermediary transfer belt 19, and the laser scanners 11 and the likeare connected with the commercial power source in the non-contact mannerto be supplied with the electric power is employed.

The heat generating elements 111 and 112 are driven by triac drivingcircuits 60 and 70. A temperature detecting element 54 provided on theback surface of the heater is connected to the ground at one end thereofand is connected to a resistor 55 at another end thereof, and is furtherconnected to an analog input port AN0 of the CPU 32 via a resistor 56.The temperature detecting element 54 has a property that a resistancevalue is lowered when a temperature thereof is high. The CPU 32 detectsthe temperature of the heater 100 by converting information on aninputted voltage value into a temperature on the basis of a temperaturetable (not shown) preset from a divided voltage of the temperaturedetecting element and the resistor 55.

An AC waveform supplied from the commercial power source 50 is detectedby a zero-cross generating circuit 57 via the AC filter 52. Thezero-cross generating circuit 57 has a constitution in which a highlevel signal is outputted when a voltage of a commercial power source 50is not more than a threshold voltage in the neighborhood of 0 V, and alow level signal is outputted in other cases. Then, into an input portPA1 of the CPU 32, a pulse signal with a period substantially equal to aperiod of the commercial power source 50 is inputted via a resistor 58.The pulse signal outputted from the zero-cross generating circuit 57 tothe CPU 32 is a zero-cross signal (“ZEROX”). The CPU 32 detects an edgewhere a zero-cross signal is changed from the high level to the lowlevel, and use the detected edge in control of the heater.

The CPU 32 determines ON-timing when the triac driving circuits 60 and70 are driven on the basis of a temperature detected by the temperaturedetecting element 54, and then outputs driving signals Drive 1 and Drive2. First, the triac driving circuit 60 as a first driving circuit forsupplying and blocking the electric power to the heat generating element111 will be described. At heater ON-timing depending on the detectedtemperature, when the CPU 32 outputs the high level signal from anoutput port PA21, the high level signal is inputted into a base of atransistor 65 via a base resistor 67, so that the transistor 65 isturned on. When the transistor 65 is turned on, a photo-triac coupler 62is in an ON state. Incidentally, the photo-triac coupler 62 is a devicefor ensuring a creepage distance between the primary side and thesecondary side, and a resistor 66 is a resistor for limiting a currentpressing through a light-emitting diode in the photo-triac coupler 62.

Resistors 63 and 64 are bias resistors for a bi-directional thyristor(triac) 61, and the triac 61 is in an electric conduction state byturning on the photo-triac coupler 62. The triac 61 is an element held,when an ON-trigger functions during the electric power supply from thecommercial power source 50, in the electric conduction state until theAC voltage becomes zero volts. As a result, the electric power dependingon the ON-timing of the triac 61 is to be supplied to the first heatgenerating member 111.

A constitution of the triac driving circuit 70 as a second drivingcircuit for supply and blocking the electric power to the second heatgenerating element 112 is the same as that of the triac driving circuit60, and therefore will be omitted from description. As described above,the first heat generating element 111 and the second heat generatingelement 112 are controlled independently of each other.

[Power Source Unit]

Next, with reference to (b) of FIG. 2, the power source unit (powersource portion) 53 (a broken line portion in (b) of FIG. 2) will bedescribed. The voltage of the commercial power source 50 is diodebridges 81 to 84 via the inlet 51 and the AC filter 52. The AC voltageinputted in the diode bridges 81 to 84 is subjected to full-waverectification and then is smoothened by a smoothing capacitor 86. Thevoltage smoothened by the smoothing capacitor 86 is inputted into aswitching power source 87 which is a DC-DC converter, so that theswitching power source 87 outputs a secondary-side voltage. In theswitching power source 87, an insulated type transducer is used forensuring electrical insulation between the primary and secondary sides.The voltage outputted from the switching power source 87 is smoothenedby the smoothing capacitor 88 and is outputted as a secondary-sidevoltage.

[Control of Electric Power Supply to Heater]

The CPU (controller) 32 sets a duty ratio (level) of the electric powerdepending on a detected temperature of the temperature detecting element54 every one control period which is a plurality of periods of an ACwaveform of the commercial power source 50. The driving circuits 60 and70 are controlled by the CPU 32 so that the electric power supplied toeach of the first and second heat generating elements 111 and 112provides the duty ratio set by the CPU 32.

Specifically, the electric power control by the CPU 32 in the case wherethe electric power of the duty ratio P=75% is intended to be supplied tothe heater 100 (in the case where a level of the total electric powersupplied to the first and second heat generating elements 111 and 112corresponds to the duty ratio of 75%) will be described using each ofoperational waveforms in 4 full-wave periods of the commercial powersource voltage. The 4 full waves of the commercial power source voltagerefers to a voltage in which corresponding to 4 periods of thecommercial power source voltage, and is 8 half waves in terms of thehalf wave. In FIG. 3, (a) shows a waveform of an AC voltage of thecommercial power source 50 (hereinafter, this voltage is referred to asa commercial power source voltage 501). In (a) of FIG. 3, each of (i) to(viii) represents the number of associated one of the 8 half waves. InFIG. 3, (b) shows a waveform of a zero-cross signal 502 outputted fromthe zero-cross generating circuit 57. In FIG. 3, (c) shows a waveform ofa current passing through the first heat generating element 111, and (d)shows a waveform of a current passing through the second heat generatingelement 112. In FIG. 3, (e) shows a resultant waveform 505 of thecurrents passing through the first and second heat generating elements111 and 112. The abscissa represents a time.

When the commercial power source voltage 501 is inputted into thezero-cross generating circuit 57, the zero-cross generating circuit 57generates zero-cross signal 502 and outputs the zero-cross signal 502 tothe CPU 32. In this embodiment, the 4 full waves constitute one controlperiod. When the current having a phase control waveform (waveform suchthat the current flows from a halfway point of the half wave) is passedthrough one of the heat generating elements every half wave, electricpower supply control such that the electric power supply of 100% (fullenergization (electric conduction)) is carried out for the other heatgenerating element or the electric power supply is not carried out(i.e., the electric power supply of 0% (non-energization) is carriedout) for the other heat generating element is effected.

In FIG. 3, each of (c) and (d) shows a waveform such that in anequiphase half wave over the periods of the one control period, thecurrent is passed through one of the heat generating elements from ahalfway point of the half wave, and is passed through or is not passedthrough the other heat generating element throughout the half waveperiod. For example, in the period of the half wave(i), the current ispassed through the first heat generating element throughout the halfwave period, and the current is passed through the second heatgenerating element from the halfway point of the half wave period. Arule of the current waveforms which pass through the first and secondheat generating elements, respectively, and which provide a relationshipbetween (c) and (d) of FIG. 3 is a first rule. In this embodiment, atleast 2 half waves of the 8 half waves in the one control periodconstitute phase-controlled waveforms. For example, the followingelectric power supply control is effected in this embodiment. That is,in the first half wave (half wave (i)) in the one control period, theelectric power is supplied to the second heat generating element 112 ina phase control manner (supplied electric power: 50%) ((d) of FIG. 3),and the electric power of 100% is supplied to the first heat generatingelement 111 ((c) of FIG. 3). The supplied electric power of 50% in eachhalf wave also means that an electric power duty ratio in each half waveis 50%. Further, in the second half wave (half wave (ii)) in the onecontrol period, the electric power is supplied to the first heatgenerating element 111 in the phase control manner (supplied electricpower: 50%) ((c) of FIG. 3), and the electric power of 100% is suppliedto the second heat generating element 112 ((d) of FIG. 3). In this way,in the periods of the one control period (periods of the half waves (i)to (viii)), the phase-controlled waveforms are alternately set for thefirst and second heat generating elements.

In the case where the electric power of 75% is supplied to the heater100, the CPU 32 determines the supplied electric power (electric powerduty ratio) every half wave so that average supplied electric power for4 full waves (half waves (i) to (viii)) is 75% for each of the first andsecond heat generating elements. That is, the CPU 32 sets the currentwaveform for each of the half waves. Hereinafter, a pattern such thatthe electric power supplied to the first heat generating element or thesecond heat generating element is set in each of the half waves in theone control period is referred to as a supplied electric power pattern.As shown in (c) of FIG. 3, the supplied electric power pattern to thefirst heat generating element 111 is such that the supplied electricpower is determined every half wave as in the current waveform 503 (inthe order of 100%, 50%, 50%, 100%, 100%, 50%, 50% and 100%). Further, asshown in (d) of FIG. 3, the supplied electric power pattern to thesecond heat generating element 112 is such that the supplied electricpower is determined every half wave as in the current waveform 504 (inthe order of 50%, 100%, 100%<50%, 50%, 100%, 100% and 50%). In ROM 32 ain the CPU 32, a table of ON-timing factors (coefficients) of controlportion the supplied electric power is stored, and the CPU 32 calculatesan ON-timing using the table stored in the ROM 32 a. The table stored inthe ROM 32 a is, e.g., as shown in Table 1. In Table 1, the left columnrepresents the duty ratio (%) of the supplied electric power, and theright column represents the ON-timing factor corresponding to the dutyratio (%). In Table 1, the ON-timing factor of 0 corresponding to thecase where a phase of the waveform of the commercial power sourcevoltage 501 is 0°, and the ON-timing factor of 1 corresponds to the casewhere the phase of the waveform of the commercial power source voltage501 is 180°. For this reason, Table 1 is also the table showingcorrespondence between the phase of the waveform of the commercial powersource voltage 501 and the duty ratio (%) of the supplied electricpower.

TABLE 1 (ON-timing table) SUPPLIED ELECTRIC POWER ON-TIMING FACTOR  0% 120% 0.664 40% 0.550 50% 0.5 60% 0.450 80% 0.336 100%  0

The CPU 32 converts the phase of the ON-timing into a time, on the basisof the table (Table 1), by multiplying a half period of the waveform ofthe commercial power source voltage 501 detected by the zero-crosssignal 502 by the ON-timing factor. For example, when a frequency of thecommercial power source voltage 501 is 50 Hz and the ON-timing forsupplying the electric power of 50% is converted into the time, theON-timing is calculated as after a lapse of 5.0 ms (milliseconds) (=20ms/2×0.5) from the timing when the zero-cross signal is detected. TheCPU 32 outputs a high-level signal to the output port PA2 or PA3 afterthe lapse of 5.0 ms from the time when the level of the zero-crosssignal 502 is changed from the high level to the low level or from thelow level to the high level. As a result, the CPU 32 supplies theelectric power of 50% to the first heat generating element 111 or thesecond heat generating element 112. In this way, by supplying theelectric power every half wave depending on the supplied electric powerpattern of each heater, it is possible to supply the electric power of75% to the heater 100 in the 4 full waves.

[Power Source Current Supply Timing and Electric Power Supply Timing toHeater]

In FIG. 4, (a) is a graph, in which the abscissa represents the time,showing the commercial power source voltage 501 inputted into the imageforming apparatus and the current passing through the surface unit(power source portion) 53. In (a) of FIG. 4, the commercial power sourcevoltage 501 is indicated by a broken line, and the current (power sourcecurrent) passing through the power source unit 53 is indicated by asolid line. Further, the left-side ordinate represents the voltage V(V), and the right-side ordinate represents the current I (A), and theseare also true for (b) to (d) of FIG. 4. In the case of the power sourceunit (power source portion) 53 in which the power factor improvingcircuit is not mounted, the current starts to flow at point B as in thecurrent waveform in (a) of FIG. 4, and the flow of the current stops atpoint A. Thus, the point B is supply starting timing, and the point A issupply ending timing. As shown in this figure, when a current conductionangle is narrowed and the voltage waveform and the current waveform aredissociated, the power factor lowers.

In FIG. 4, (b) is a graph, in which the abscissa represents the time,showing the commercial power source voltage 501 inputted into the imageforming apparatus and a waveform showing a heater resultant current(total current passing through the first and second heat generatingelements). The heater resultant current in this figure is a current inthe case of a supplied electric power pattern in which when the electricpower is supplied to one of the heat generating elements by phasecontrol, the electric power of 100% or 0% is supplied to the other heatgenerating element. The heater resultant current waveform shown in (b)of FIG. 4 corresponds to the resultant current waveform 505 shown in (e)of FIG. 3. In FIG. 4, (b) is the graph showing the resultant currentwaveform in the case where the current having the phase-controlledwaveform is started to be passed through the first heat generatingelement or the second heat generating element from the supply endingtiming (timing of A in (a) of FIG. 4) of the power source current. Forexample, in the case where the electric power supply control is effectedin the supplied electric power pattern as shown in FIG. 3, in the firsthalf wave (half wave (i)) in the one control period, the electric powerof 100% is supplied to the first heat generating element 111, and theelectric power of 50% is supplied to the second heat generating element112. The electric power is supplied, from the phase of 90° to the phaseof 180° (in the case where the duty ratio of 50%, the electric powersupply start phase is 90°), to the second heat generating element 112which is the heat generating element through which the current of thephase-controlled waveform is to be passed, and is not supplied from thephase of 0° to the phase of 90°. In this way, in the case where thecontrol as shown in FIG. 3 is effected, there is a period (e.g., fromthe phase of 0° to the phase of 90°) in which the heat generatingelement through which the current of the phase-controlled waveform is tobe passed is turned off, and therefore the voltage waveform and thecurrent waveform are dissociated from each other, so that the powerfactor lowers.

In FIG. 4, (c) is a graph showing the commercial power source voltage501 inputted into the image forming apparatus and the waveform of ainlet current. The inlet current is a resultant current of the currentpassing through the power source unit (predetermined portion) 53 and thecurrent passing through the heater 100. The power source current shownin (a) of FIG. 4 and the heater resultant current shown in (b) of FIG. 4are synthesized, so that the inlet current approaches the commercialpower source voltage 501 and thus the power factor is improved. In thisway, in synchronism with the supply ending timing (timing of A) of thepower source current, the supply of the electric power of thephase-controlled waveform to the heat generating element is started, sothat a high power factor can be achieved.

In FIG. 4, (d) is a graph showing an inlet current waveform in the casewhere the supply of the electric power of the phase controlled waveformto the heat generating element at timing substantially the same as thesupply starting timing (timing of B in (a) of FIG. 4) of the powersource current. In this case, the current abruptly fluctuates at thetiming of B. Accordingly, the dissociation is generated between theinlet current waveform and the commercial power source voltage 501, sothat the power factor lowers.

[Relationship Between Supplied Electric Power and Power Factor]

In FIGS. 5, (a) and (b) are graphs each showing a relationship betweenthe duty ratio of the supplied electric power and the power factor. InFIG. 5, (a) is the graph, in which the abscissa represents the dutyratio of the supplied electric power, showing the power factor in asupplied electric power pattern in Comparison Example in which when theelectric power is supplied to one of the heat generating elements in thephase-controlled manner, the electric power of 100% or 0% is supplied tothe other heat generating element. In (a) of FIG. 5, in the neighborhoodof point C, i.e., in the neighborhood of the point where the duty ratioof the supplied electric power is 70%, the power factor is improved. Thepoint C is a supplied electric power point where the supply startingtiming of the electric power by the phase control coincides with thesupply ending timing of the power source current. The supplied electricpower pattern is, e.g., as shown in (a) of FIG. 6. In FIG. 6, (a) showsthe duty ratio of the supplied electric power in each of half waves foreach of the first and second heat generating elements 111 and 112 andshows the supplied electric power duty ratio in the 8 half wavesconstituting the one control period. In (a) of FIG. 6, P represents apositive half wave, and N represents a negative half wave. Further, afirst full wave P corresponds to a first half wave (half wave (i)), anda second full wave N corresponds to a second half wave (half wave (ii)).These are also true for tables of (b) to (d) of FIG. 6.

In the supplied electric power pattern shown in (a) of FIG. 6, over theperiods of the one control period, when the electric power 100% issupplied to one heat generating element, the electric power of 40% issupplied to the other heat generating element in the phase-controlledmanner, and the phase-controlled electric power supply is started in theneighborhood of the supply ending timing of the power source current(waveform of rule 1). In the supplied electric power pattern shown in(a) of FIG. 6, an average supplied electric power duty ratio is 70%. Inthe case where the electric power is supplied in the supplied electricpower pattern shown in (a) of FIG. 6, the inlet current waveform is asshown in (c) of FIG. 4 and approaches the commercial power sourcevoltage 501, and therefore the power factor is improved.

On the other hand, in (a) of FIG. 5, in the neighborhood of point D,i.e., in the neighborhood of the point where the duty ratio of thesupplied electric power is 85%, the power factor is lowered. The point Dis a supplied electric power point where the supply starting timing ofthe electric power by the phase control for one of the heat generatingelements coincides with the supply starting timing of the power sourcecurrent. The supplied electric power pattern is, as shown in (b) of FIG.6.

Also in the supplied electric power pattern shown in (b) of FIG. 6, overthe periods of the one control period, when the electric power 100% issupplied to one heat generating element, the electric power of 70% issupplied to the other heat generating element, and the phase-controlledelectric power supply is started in the neighborhood of the supplystarting timing of the power source current (waveform of rule 1). In thesupplied electric power pattern shown in (b) of FIG. 6, an averagesupplied electric power duty ratio is 85%. In the case where theelectric power is supplied in the supplied electric power pattern shownin (a) of FIG. 6, the inlet current waveform is as shown in (d) of FIG.4 and is dissociated from the commercial power source voltage 501, andtherefore the power factor is lowered. In this way, in the case wherethe electric power is supplied by the waveform of the rule 1, the powerfactor is high when the duty ratio of the total electric power suppliedto the first and second heat generating elements is about 70%, but islowered when the duty ratio is about 85%. In this embodiment, aconstitution in which the half wave such that the electric power supplyto the heat generating element is started in the neighborhood of thesupply starting timing of the power source current is decreased and inwhich the half wave such that the electric power supply to the heatgenerating element is started in the neighborhood of the supply endingtiming of the power source current is increased is employed. As aresult, in this embodiment, a high power factor is realized. That isdepending on the duty ratio of the total electric power supplied to thefirst and second heat generating elements, the rule of the AC waveformpassed through the first and second heat generating elements.

[Supplied Electric Power Pattern in this Embodiment]

Next, the supplied electric power pattern in this embodiment will bespecifically described with reference to (b) and (c) of FIG. 6. In thisembodiment, as in the supplied electric power pattern in (c) of FIG. 6,the half wave in which the duty ratio is 70% in the supplied electricpower pattern in (b) of FIG. 6 is replaced, in the neighborhood of thesupply ending timing of the power source current, with the half wave inwhich the duty ratio is 40% where the power factor is good or the halfwave in which the duty ratio is 100% where the power factor isoriginally good. In the supplied electric power pattern shown in (c) ofFIG. 6, average supplied electric power is 85% similarly as in thesupplied electric power pattern shown in (b) of FIG. 6. In the suppliedelectric power pattern in (c) of FIG. 6, a waveform of a second rule inwhich in an equiphase half wave of at least a part of the periods of theone control period, the current is passed through both of the first andsecond heat generating elements throughout the half wave period.

For example, in the first full wave P (half wave (i)) in (b) of FIG. 6,the electric power of 70% in duty ratio is supplied to the second heatgenerating element 112, and on the other hand, in the first full wave P(half wave (ii)) in (c) of FIG. 6, the electric power of 40% in dutyratio is supplied to the second heat generating element 112. Further, inthe first full wave N (half wave (ii)) in (b) of FIG. 6, the electricpower of 70% in duty ratio is supplied to the first heat generatingelement 111, and on the other hand, in the first full wave N (half wave(ii)) in (c) of FIG. 6, the electric power of 100% in duty ratio issupplied to the first heat generating element 111. In this embodiment,by employing such a constitution, even in the case where the averagesupplied electric power to the entire heater 100 in the one controlperiod is 85%, it becomes possible to achieve a high power factor.

In FIG. 5, (b) is a graph showing the power factor in the suppliedelectric power pattern in Comparison Example indicated by a solid lineand the power factor in the supplied electric power pattern inEmbodiment 1 indicated by a broken line. In the neighborhood of thepoint D in (b) of FIG. 5, it is understood that the power factor in thesupplied electric power pattern in Embodiment 1 is improved comparedwith the case of Comparison Example. In this embodiment, the high powerfactor is achieved by starting the electric power supply by the phasecontrol at the supply ending timing of the power source current in thesupplied electric power pattern in the 4 full-wave period and by usingthe duty ratio of 100% where the power factor is originally good. Inaddition to this method, the following manner may also be used. That is,by prolonging the length of the one control period, the power factor canbe improved even at the duty ratio where the power factor is lowered asin the case of point E in (b) of FIG. 5. For example, the one controlperiod is constituted by 8 full-wave period (i.e., the one controlperiod is constituted by 16 half waves), and thus the supplied electricpower pattern at the point C and the supplied electric power pattern atthe point D are combined, so that the power factor at the point E can beimproved.

As the supply ending timing of the power source current in thisembodiment, the supply ending timing of the power source currentprepared in advance is used. However, the supply ending timing of thepower source current changes also depending on the power source voltageand the power source current, and therefore the supply ending timing mayalso be changed depending on the commercial power source voltage 501,the inlet current, a sequence of the image forming apparatus, and thelike, and is not limited to that in this embodiment.

As described above, by using the supplied electric power pattern basedon the supply ending timing of the power source current, it becomespossible to realize the high power factor even in the power sourceprovided with no power factor improving circuit. The number of the heatgenerating elements for the heater 100, the length of the one controlperiod and the setting method of the supplied electric power are notlimited to those described in this embodiment.

[Power Control Process for Improving Power Factor]

FIG. 7 is a flowchart for illustrating a sequence for setting thesupplied electric power pattern for improving the power factor. In FIG.6, (d) is a basic table of the supplied electric power pattern in thisembodiment and is stored in, e.g., the ROM 32 a. The basic table in (d)of FIG. 6 is constituted by the following 3 components. First, the basictable of (d) of FIG. 6 is constituted by Piso(0) in which the suppliedelectric power is 100% (“AE” (all energization) in the figure) or 0%(“NE” (non-energization) in the figure). Further, the basic table in (d)of FIG. 6 is constituted by Piso(1) in which the electric power issupplied by first phase control (“PHS1” (phase 1)) and Piso(2) in whichthe electric power is supplied by second phase control (“PHS2” (phase2)). By using the basic table in (d) of FIG. 6, the process for settingthe supplied electric power pattern for improving the power factor willbe described along the flowchart.

In step S101, the CPU 32 calculates a duty ratio P (%) of the suppliedelectric power to the heater 100 (i.e., the duty ratio of the totalelectric power supplied to the first and second heat generatingelements) on the basis of a set temperature of the heater 100, i.e., atarget temperature in temperature control, and a present temperaturedetected by the temperature detecting element 54. The duty ratio P (%)calculated by the CPU in S101 is average supplied electric powersupplied to the entire heater 100 in the one control period, and, e.g.,in the case of (c) of FIG. 6, the duty ratio P is 85%. In S102, the CPU32 calculates a duty ratio Pa of the phase-controlled waveform in whichthe power factor is most improved (hereinafter, referred to as a powerfactor-improving duty ratio), from the supply ending timing of the powersource current prepared in advance. The duty ratio P in the duty ratioin a one control period unit, and on the other hand, the duty ratio Pais the duty ratio in a half wave unit. As described with reference to(b) and (c) of FIG. 6, in the case where the supplied electric power (P%) is 85%, the power factor is lowered when the supplied electric powerin one half wave is 70% and is improved when the supplied electric powerin one half wave is 40%. In such a case, the CPU 32 calculates the powerfactor-improving duty ratio Pa as 40%. Also in the case where thecalculated duty ratio P is less or more than 85%, not 85%, Pa is set at40%. In this embodiment, the supply ending timing of the power sourcecurrent prepared in advance is stored in, e.g., the ROM 32 a.

In S103, the CPU 32 discriminates whether or not the duty ratio P is 50%or more. In the case where the CPU 32 discriminates that the duty ratioP is 50% or more in S103, the CPU 32 sets Piso(0) at 100% (Piso(0)=100)in S104. In the case where the CPU 32 discriminates that the duty ratiois not 50% or more in S103, the CPU 32 sets Piso(0) at 0% (Piso(0)=0) inS105. For example, in the case where the duty ratio P calculated by theCPU 32 in S101 is 85%, Piso(0)=100 is set. In S106, the CPU 32 setsPiso(1) at the power factor-improving duty ratio Pa calculated in S102(Piso(1)=Pa), and sets Piso(2) at 100% (Piso(2)=100) in S107. Forexample, in the case where the power factor-improving duty ratio Pacalculated by the CPU 32 in S102 is 40%, Piso(1)=40 is set.

In S108, the CPU 32 discriminates whether or not a duty ratio of averagesupplied electric power in the 4 full waves set by using the duty ratioPa (i.e., temperature duty ratio=((Piso(0)×2+Piso(1)+Piso(2))/4) isequal to the duty ratio P calculated in S101. As shown in (c) of FIG. 6,of the 4 full waves, the later 2 full waves have a pattern in whichcomponents of the early 2 full waves are reversed, and therefore theduty ratio can be calculated by a Formula: (Piso(0)×2+Piso(1)+Piso(2))/4(4:the number of the half waves), and this is true for subsequentcalculations. In S108, in the case where the CPU 32 discriminates thatthe temporary duty ratio is not equal to the duty ratio P, the CPU 32discriminates in S109 whether or not the temporary duty ratio is largerthan the duty ratio P. For example, in the case where the duty ratio Pcalculated in S101 is 83%, not 85%, the duty ratio P is not equal to thetemporary duty ratio, and therefore, the sequence transfers from S108and S109. In S109, in the case where the CPU 32 discriminates that thetemporary duty ratio is larger than the duty ratio R, the sequence goesnot a process of S110, and in the case where the CPU 32 discriminatesthat the temporary duty ratio is not larger than the duty ratio P, i.e.,smaller than the duty ratio P, the sequence goes to a process of S113.In the case where the sequence transfers from S108 and S109, thetemporary duty ratio calculated as Piso(1)=Pa does not coincide with theduty ratio P. Therefore, there is a need to correct Piso(1) in orderthat the temporary duty ratio coincides with the duty ratio P. In thecase where Piso(1) is not equal to Pa, the supply starting timing of thecurrent of the phase-controlled waveform is deviated from the supplyending timing (A in FIG. 4) of the power source current, but when adeviation amount is smaller than that with respect to the timing shownin (d) of FIG. 4, the lowering in power factor can be suppressed.Accordingly, Piso(1) is a correctable value, and a threshold X describedbelow is an upper limit of the correction of Piso(1) in consideration ofthe power factor.

In S110, the CPU 32 power sources whether or not Piso(1) is smaller thana value (Pa−X) obtained by subtracting a predetermined duty ratio X (%)from the power factor-improving duty ratio Pa. The threshold X is avalue of 0-25, preferably a value of 0-15. In S110, in the case wherethe CPU 32 discriminates that Piso(1) is smaller than the value (Pa−X),the sequence goes to a process of S111. In S110, in the case where theCPU 32 discriminates that Piso(1) is not smaller than the value (Pa−X),the sequence goes to process of S112. In S106, Piso(1)=Pa is set, andtherefore in the case where the sequence first goes to S110, thediscrimination in S110 is “No”, so that the sequence goes to S112.

In S111, the power factor is rather lowered when the value is furthersubtracted from Piso(1), and therefore the CPU 32 does not subtract thevalue from Piso(1), but Piso(2) is decreased by 1% (Piso(2)=Piso(2)−1%).On the other hand, in S112, the CPU 22 decreases Piso(1) by 1%(Piso(1)=Piso(1)−1%). Further, in S113, the CPU 32 increases Piso(1) by1% (Piso(1)=Piso(1)+1%). In the case where the CPU 32 makes thediscrimination of “No” in S109, the value cannot be added toPiso(2)=100, and therefore the CPU 32 does not make the discriminationas in S110 but performs the process of S113. In the case where the CPU32 discriminates that the temporary duty ratio is equal to the dutyratio P in S108, the process is ended. For example, in the case wherethe duty ratio P is 85% and the duty ratio Pa is 40%, the temporary dutyratio is 85% (=(100×2+40+100)/4), and is equal to the duty ratio P, andtherefore the setting process of the supplied electric power pattern isended.

In this embodiment, the control sequence, the tables and the circuitstructure are not limited to those described above. By the electricpower control in this embodiment, the supplied electric power patternbased on the supply ending timing of the power source current is set, sothat it becomes possible to realize the high power factor even in thepower source with no power factor-improving circuit. In this embodiment,the supplied electric power duty ratio P is calculated from the targettemperature for effecting the temperature control and the presenttemperature detected by the temperature detecting element 54, and thenthe supplied electric power pattern is set. However, a constitution inwhich an optimum supplied electric power pattern is selected on thebasis of the power source current supply ending timing from a pluralityof tables of the supplied electric power patterns corresponding tocombinations of predetermined duty ratios P with candidates forpredetermined duty ratios Pa may also be employed. In this way, thesetting method of the supplied electric power pattern is not limited tothe method described in this embodiment.

Another Embodiment

In the above-described driving circuit for the heater 100, the triac isused. In the case where the triac is used, as described above, aconstitution in which the supply ending timing of the power sourcecurrent and the supply starting timing of the electric power by thephase control coincide with each other is employed. This embodiment isnot limited to the driving circuit using the triac, but can also beapplied to a driving circuit using, e.g., a field-effect transistor(FET).

In the case where the FET is used in the driving circuit for the heater100, the power factor is improved when ON-timing or OFF-timing of theFET is controlled in the following manner. When description is made withreference to (a) of FIG. 4, the FET is turned on at timing of a phase of0°, and is turned off at timing of B. Then, the FET is turned on attiming of A, i.e., supply ending timing of the power source current, andis turned off at timing of the phase of 180°. For this reason, in thecase where the present invention is applied to the driving circuit usingthe FET, for setting the supplied electric power pattern, not only thesupply ending timing of the power source current but also the supplystarting timing of the power source current are needed. The supplystarting timing of the power source current may only be required to bestored in advance in, e.g., the ROM 32 a similarly as in the case of thesupply ending timing of the power source current described above. Thisis true for Embodiment 2.

As described above, according to this embodiment, it is possible toimprove the power factor while realizing downsizing of the image formingapparatus and cost reduction.

Embodiment 2

In Embodiment 1, the example in which the supplied electric powerpattern in which the power factor is improved on the basis of the powerfactor-improving duty ratio Pa calculated from the supply ending timingof the power source current is set and in which the electric powersupply to the heater 100 is carried out in the supplied electric powerpattern in one species of the control period was described. In general,a maximum current which can be supplied from the commercial power source50 into the image forming apparatus is limited by standard, so that ahigh power factor is required only in the case where the current of theinlet 51 is in the neighborhood of the maximum current standard.Further, in order to minimize a temperature ripple as seen in the heater100, the control period of the electric power supply is required to beshortened. Therefore, in Embodiment 2, an example in which whether ornot control for improving the power factor should be effected isdiscriminated depending on a detection result of the inlet current andthen the control period of the electric power supply and the suppliedelectric power pattern are switched will be described. The structures ofthe image forming apparatus and the fixing device 30 are similar tothose in Embodiment 1, and therefore a difference from Embodiment 1 willbe principally described, and common constitutions will be omitted fromdescription by adding the same reference numerals or symbols.

[Inlet Current Detecting Circuit]

An inlet current detecting circuit in a heater driving circuit in thisembodiment will be described with reference to (a) of FIG. 8. Thecurrent flowing into the inlet 51 is inputted into an inlet currentdetecting circuit 181 via a current transducer 180. In the inlet currentdetecting circuit 181, the inputted current is converted into a voltageand then is outputted. A current detection signal (“HCRRT1”) which isobtained by converting the current into the voltage by the inlet currentdetecting circuit 181 and which is outputted from the circuit 181 isinputted into an input port PA4 of the CPU 32 via a resistor 182, andthen is A/D-converted, so that the signal is controlled as a digitalvalue.

In FIG. 8, (b) is a block diagram for illustrating a constitution of theinlet current detecting circuit 181 in this embodiment. FIG. 9 is awaveform chart for illustrating an operation of the inlet currentdetecting circuit 181 in this embodiment.

Specifically, in FIG. 9, (a) shows a current waveform 701 of an inletcurrent I2 supplied via the inlet 51 and the AC filter 52, and the inletcurrent I2 is converted into the voltage into the secondary side by thecurrent transducer 180. The inlet current I2 is a resultant current of aheater current I1 passing through the heat generating elements 111 and112 and a current I3 passing through the power source unit 53 (alsoreferred to as a power source current I3). In FIG. 9, (b) shows acurrent waveform 702 of the heater current I1 (passing through the heatgenerating elements 111 and 112. In FIG. 9, (c) shows a waveform of thezero-cross signal 502 outputted from the zero-cross generating circuit57. In these figures, the abscissa represents the time.

The voltage outputted from the current transducer 180 is rectified bydiodes 301 and 303 of the inlet current detecting circuit 181 shown in(b) of FIG. 8. Resistors 302 and 305 are load resistors. A voltagewaveform 703 is a voltage waveform which is subjected to half-waverectification by the diode 303, and the voltage waveform 703 is inputtedinto a multiplier 306 via the resistor 305 shown in (b) of FIG. 8. Awaveform 704 shown in (e) of FIG. 9 is a voltage waveform squared by themultiplier 306. The squared voltage waveform 704 is inputted into (−)terminal of an operational amplifier 309 via a resistor 307 in (b) ofFIG. 8. On the other hand, into (+) terminal of the operationalamplifier 309, a reference voltage 317 is inputted via a resistor 308,so that the reference voltage 317 is inverted and amplified by afeedback resistor 310. Incidentally, the operational amplifier 309 issupplied with the electric power from one of the power sources. Thewaveform inverted and amplified on the basis of the reference voltage317, i.e., the waveform 705, shown in (f) of FIG. 9, which is an outputof the operational amplifier 309 is inputted into (+) terminal of anoperational amplifier 312. The inlet current detecting circuit alsoincludes the resistor 304 and a buffer 316. A reference potential of thecurrent transducer 180 is determined from a reference voltage 317 viathe buffer 316.

The operational amplifier 312 controls a transistor 313 so that acurrent determined by a voltage difference between the reference voltage317 inputted into (−) terminal thereof and the waveform inputted into(+) terminal thereof, and a resistor 311 is caused to flow into acapacitor 314. The capacitor 314 is charged with the current detected bythe voltage difference between the reference voltage 317 inputted into(−) terminal and the waveform inputted into (+) terminal of theoperational amplifier 312, and the resistor 311.

When a half-wave rectification section by the diode 303 is ended, thereis no charging current to the capacitor 314, and therefore a resultantvoltage value V2f is peak-held as shown in a waveform 706 in (g) of FIG.9.

Here, a transistor 315 is turned on in a half-wave rectification period,so that the charging current of the capacitor 314 is discharged. Thetransistor 315 is turned on and off by a DIS signal outputted from theCPU 32 shown in (h) of FIG. 9 as a waveform 707. On the basis of thezero-cross signal shown in (c) of FIG. 9 as the waveform 502, the CPU 32controls the transistor 315. The DIS signal outputted from the CPU 21becomes a high level after a lapse of a predetermined time Tdly from arising edge of the zero-cross signal, and becomes low level at a fallingedge of the zero-cross signal 502. As a result, the CPU 32 is capable ofcontrolling the inlet current detecting circuit 181 without interferingwith a heater current period in the half-wave rectification period ofthe diode 303.

That is, the peak-holding voltage V2f of the capacitor 314 is anintegrated value of a squared value, in a half period, of the waveformwhich is voltage-converted, from the current waveform in the secondaryside, by the current transducer 180. Then, the voltage of the capacitor314 is outputted, as HCRRT1 signal shown in the waveform 706, in (g) ofFIG. 9, from the inlet current detecting circuit 181 to the CPU 32. TheCPU 32 subjects the HCRRT1 signal 706, inputted from the input port PA4,to A/D conversion until the lapse of the predetermined time Tdly fromthe rising edge of the zero-cross signal 502. The inlet current I2subjected to the A/D conversion is a current value for a full wave ofthe commercial power source voltage, and then the CPU 32 averages theinlet current I2 for 4 full waves of the commercial power source voltageand calculates electric power, to be consumed by the entire apparatus,by multiplying the average value by a coefficient prepared in advance.However, the detecting method of the inlet current I2 is not limitedthereto.

In this embodiment, in the case where the inlet current I2 exceeds apredetermined current I4, the CPU 32 discriminates that there is a needto improve the power factor and sets the supplied electric power patternbased on the supply ending timing of the power source current I3 in the4 full-wave periods described in Embodiment 1. The predetermined currentI4 is a value determined in advance on the basis of the inlet currentstandard. On the other hand, in the case where the inlet current I2 isnot more than the predetermined current I4, the CPU 32 discriminatesthat there is no need to improve the power factor, and gives highpriority to shortening of the control period of the electric powersupply, so that the CPU 32 sets the supplied electric power pattern inthe 2 full-wave periods. In the case where the priority is given to theshortening of the control period, the one control period may only berequired to be shorter than the supplied electric power pattern (e.g., 8half waves) for improving the power factor and may only be required toinclude at least two half waves for effecting the phase control.

In this embodiment, switching detection of the supplied electric powerpattern is made on the basis of the detection result of the inletcurrent detecting current 181. However, e.g., as in a constitution inwhich the supplied electric power pattern is switched depending on thecommercial power source voltage or in a constitution in which thesupplied electric power pattern is switched depending on an operation ina mode such as warm-up made or a print mode, the discrimination is notlimited to that in this embodiment. Further, a constitution in which theswitching between the supplied electric power pattern based on thesupply ending timing of the power source current I3 in the 4 full-waveperiods and the supplied electric power pattern based on the supplyending timing of the power source current I3 in the 8 full-wave periodsis made depending on the control period or the like of required electricpower control may also be employed. In this way, the type and the numberof the supplied electric power patterns to be switched are not limitedto those in this embodiment.

[Electric Power Control Process for Improving Power Factor]

The control process in this embodiment will be described along aflowchart of FIG. 10. In this embodiment, the steps in which the sameprocesses as those described in Embodiment 1 are represented by the samestep symbols and will be omitted from description. In S201, the CPU 32calculates the average inlet current I2 in the 4 full-wave periods byusing the inlet current detecting circuit 181 described with referenceto FIGS. 8 and 9. In S202, the CPU 32 compares the inlet current I2 withthe predetermined current I4 determined in advance on the basis of theinlet current standard, and thus periods whether or not the inletcurrent I2 is larger than the predetermined current I4. In S202, in thecase where the CPU 32 discriminates that the inlet current I2 is notlarger than the predetermined current I4, in S204, the CPU 32 sets thesupplied electric power pattern in the second full-wave periods (2periods) prepared in advance for the supplied electric power pattern.

On the other hand, in S202, in the case where the CPU 32 discriminatesthat the inlet current I2 is larger than the predetermined current I4,in S203, the CPU 32 sets the control period of the electric power supplyat the 4 full-wave periods. Then, the CPU 32 carries out the processesfrom S102 and S113 as described above, so that the CPU 32 sets thesupplied electric power pattern based on the supply ending timing of thepower source current I3 in the 4 full waves. In this way, by performingthe supplied electric power pattern setting process as shown in FIG. 10,in the case where the inlet current I2 is larger than the predeterminedcurrent I4, the supplied electric power pattern based on the supplyending timing of the power source current I3 is set. As a result, itbecomes possible to realize the high power factor even in the powersource with no power factor-improving circuit. On the other hand, in thecase where it can be detected that there is no improvement in powerfactor when the inlet current I2 is smaller than the predeterminedcurrent I4, in order to minimize the temperature ripple as seen in theheater 100, it is possible to give high priority to shortening of thecontrol circuit of the electric power control.

As described above, according to this embodiment, the power force can beimproved while realizing downsizing of the image forming apparatus andcost reduction.

Embodiment 3

In Embodiments 1 and 2, the supply starting timing of the power sourcecurrent I3 and the supply ending timing of the power source current I3are prepared in advance. In Embodiment 3, a constitution applicable toeven the case where the supply starting timing of the power sourcecurrent I3 and the supply ending timing of the power source current I3change depending on variations or the like in capacity of the commercialpower source voltage 501 and the primary smoothing capacitor 86 will bedescribed. That is, a method in which the supply starting timing of thepower source current I3 and the supply ending timing of the power sourcecurrent I3 can be detected without providing a dedicated detectingcircuit even in the case where the supply starting timing of the powersource current I3 and the supply ending timing of the power sourcecurrent I3 change will be described. Also in this embodiment, adifference from Embodiments 1 and 2 will be principally described, andcommon constitutions will be omitted from description by adding the samereference numerals or symbols. In this embodiment, a method in which thesupply starting timing of the power source current I3 and the supplyending timing of the power source current I3 are detected using theinlet current detecting circuit 181 and the heater driving circuit shownin (a) of FIG. 8 which have already existed will be described. Theelectric power control process for improving the power factor is similarto those in Embodiments 1 and 2, and will be omitted from description.

[Relationship Among Heater Current, Inlet Current and Supplied ElectricPower to Heater]

FIG. 11 is a graph showing a relationship between the duty ratio of theelectric power supplied to the heater and each of the squared value(thick solid line) of an effective value of the inlet current I2 and asquared value (thin solid line) of an effective value of the heatercurrent I1. In FIG. 11, the abscissa represents the duty ratio of theelectric power supplied to the heater in one half wave. It is understoodthat the squared value of the heat current I1 is increased with acertain slope when the duty ratio is increased. This can be similarlysaid from also the following formula 1:

$\begin{matrix}{{I\; 1^{2}} = \frac{P}{R}} & \left( {{formula}\mspace{14mu} 1} \right)\end{matrix}$

In the above formula, P represents the supplied electric power, and Rrepresents a resistance value of the heater 100.

On the other hand, the squared value of the inlet current I2 isdifferent in slope every region of the duty ratio when the duty ratio ofthe electric power supplied to the heater is increased. In theneighborhood of the duty ratio of 0% to 40% and the duty ratio of 75% to100%, the slope is the same as the slope of the squared value of theheater current I1. However, in a range from 40% to 75% in duty ratio, itis understood that the slope (broken line) of the inlet current I2 isabrupt compared with other ranges. Further, the region where the dutyratio is 40% to 75% is an overlapping region with the power sourcecurrent I3. That is, at a point (duty ratio of 40% in FIG. 12) where theslope of the inlet current I2 changes in an increasing direction, thetiming is the supply ending timing of the power source current I3.Further, at a point (duty ratio of 75% in FIG. 12) where the slope ofthe inlet current I2 changes in a decreasing direction, the timing isthe supply starting timing of the power source current I3. The dutyratio of 0% corresponds to the phase of 180°, and the duty ratio of 100%corresponds to the phase of 0°.

Next, with reference to FIG. 12 and formulas 2 to 6-2, a phenomenon suchthat the slope of the squared value of the inlet current I2 becomesabrupt only in the overlapping region with the power source current I3in FIG. 11 will be described. In FIG. 12, (a) to (c) are graphs eachshowing the inlet current I2, in which the abscissa represents the time,and the ordinate represents a current value. The inlet current I2 isrepresented by formula 2 below, and a squared value thereof isrepresented by formula 3 below. In the formulas 2 and 3, i2(t)represents an instantaneous value of the current passing through theinlet 51.

$\begin{matrix}{{I\; 2} = \sqrt{\frac{1}{T}{\int_{0}^{T}{i\; 2(t)^{2}\ {t}}}}} & \left( {{formula}\mspace{14mu} 2} \right) \\{{I\; 2^{2}} = {\frac{1}{T}{\int_{0}^{T}{i\; 2(t)^{2}\ {t}}}}} & \left( {{formula}\mspace{14mu} 3} \right)\end{matrix}$

(Case of (a) in FIG. 12)

Calculation of the inlet current I2 in the case where the supplystarting timing of the electric power supplied to the heater 100 islater than the supply ending timing of the power source current I3 willbe described using the formulas 2 and 3. In FIG. 12, (a) is the graphshowing the inlet current I2 in the case where the supply startingtiming of the electric power supplied to the heater 100 is later thanthe supply ending timing of the power source current I3. In (a) of FIG.12, a is the supply starting timing of the power source current I3, b isthe supply ending timing of the power source current I3, and c is thesupply starting timing of the electric power supplied to the heater 100.Further, T represents timing corresponding to the phase of 180°. At thistime, the inlet current I2 in this case can be represented by formula 4below. In the formula 4, i1(t) is in instantaneous value of the currentpassing through the heater, and i3(t) is an instantaneous value of thepower source current I3.

$\begin{matrix}{{I\; 2^{2}} = {{\frac{1}{T}{\int_{a}^{b}{i\; 3(t)^{2}{t}}}} + {\frac{1}{T}{\int_{c}^{T}{i\; 1(t)^{2}\ {t}}}}}} & \left( {{formula}\mspace{14mu} 4} \right)\end{matrix}$

When the supply starting timing c of the electric power supplied to theheater is changed in a section from b to T, it is understood that thesquared value of the inlet current I2 is changed correspondingly to achange in squared value of the heater current I1 in the second term ofthe formula 4. In (a) of FIG. 12, the section from b to T corresponds toa section from 0% to 40% in duty ratio in FIG. 11. That is, in the caseof (a) of FIG. 12, as also shown in FIG. 11, the slope of the inletcurrent I2 and the slope of the heater current I1 are equal to eachother.

(Case of (b) in FIG. 12)

Calculation of the inlet current I2 in the case where the supplystarting timing of the electric power supplied to the heater 100 islater than the supply starting timing of the power source current I3 andearlier than the supply ending timing of the power source current I3will be described with reference to (b) of FIG. 12. In (b) of FIG. 12, ais the supply starting timing of the power source current I3, b is thesupply ending timing of the power source current I3, and c is the supplystarting timing of the electric power supplied to the heater 100. Theinlet current I2 in this case can be represented by formula 5 below.

$\begin{matrix}{{I\; 2^{2}} = {{\frac{1}{T}{\int_{a}^{c}{i\; 3(t)^{2}{t}}}} + {\frac{1}{T}{\int_{c}^{b}{\left\{ {{i\; 3(t)} + {i\; 1(t)}} \right\}^{2}{t}}}} + {\frac{1}{T}{\int_{b}^{T}{i\; 1(t)^{2}{t}}}}}} & \left( {{formula}\mspace{14mu} 5} \right) \\{{I\; 2^{2}} = {{\frac{1}{T}{\int_{a}^{c}{i\; 3(t)^{2}{t}}}} + {\frac{1}{T}{\int_{c}^{b}{i\; 3(t)^{2}{t}}}} + {\frac{1}{T}{\int_{c}^{b}{i\; 1(t)^{2}{t}}}} + {\frac{2}{T}{\int_{c}^{b}{\left\{ {i\; 3(t) \times i\; 1(t)} \right\} {t}}}} + {\frac{1}{T}{\int_{b}^{T}{i\; 1(t)^{2}{t}}}}}} & {\text{(}{formula}\mspace{14mu} 5\text{-}1\text{)}} \\{{I\; 2^{2}} = {{\frac{1}{T}{\int_{a}^{b}{i\; 3(t)^{2}{t}}}} + {\frac{1}{T}{\int_{C}^{T}{i\; 1(t)^{2}{t}}}} + {\frac{2}{T}{\int_{c}^{b}{\left\{ {i\; 3(t) \times i\; 1(t)} \right\} {t}}}}}} & \left( {{formula}\mspace{14mu} 5\text{-}2} \right)\end{matrix}$

In the formula 5, a square of a to c (the first term) is a square ofonly the instantaneous value i3(t) of the power source current I3. Asection of c to be (the second term is a resultant current section ofthe instantaneous value i3(t) of the power source current I3 and aninstantaneous value i1(t) of the heater current I1. Further, in theformula 5, a section of b to T (the third term) is a section of only theinstantaneous value i1(t) of the heater current I1. When the formula 5is developed, the formula 5-1 is obtained. When the formula 5-1 issummarized, the squared value of the inlet current I2 can be expressedby the formula 5-2. In the formula 5-2, the first term represents thesquared value of the power source current I3 in a section of a to b, andthe second term represents the squared value of the heater current I1 ina section of c to T. In the formula 5-2, the third term represents theterm generated by synthesizing the instantaneous value i3(t) of thepower source current I3 and the instantaneous value i1(t) of the heatercurrent I1 and then by squaring the resultant value. When the supplystarting timing c of the electric power supplied to the heater ischanged in a section from a to b, it is understood that the squaredvalue of the inlet current I2 is changed correspondingly to a change thethird term in addition to the change in the squared value of the heatercurrent I1 in the second term of the formula 5-2. In (b) of FIG. 12, thesection from a to b corresponds to a section from 40% to 75% in dutyratio in FIG. 11. That is, in the case of (b) of FIG. 12, as also shownin FIG. 11, the slope of the inlet current I2 is more abrupt than theslope of the heater current I1.

(Case of (c) in FIG. 12)

Calculation of the inlet current I2 in the case where the supplystarting timing of the electric power supplied to the heater 100 isearlier than the supply ending timing of the power source current I3will be described with reference to (c) of FIG. 12. In (c) of FIG. 12, ais the supply starting timing of the power source current I3, b is thesupply ending timing of the power source current I3, and c is the supplystarting timing of the electric power supplied to the heater 100. Theinlet current I2 in this case can be represented by formula 6 below.

$\begin{matrix}{{I\; 2^{2}} = {{\frac{1}{T}{\int_{c}^{a}{i\; 1(t)^{2}{t}}}} + {\frac{1}{T}{\int_{a}^{b}{\left\{ {{i\; 3(t)} + {i\; 1(t)}} \right\}^{2}{t}}}} + {\frac{1}{T}{\int_{b}^{T}{i\; 1(t)^{2}{t}}}}}} & \left( {{formula}\mspace{14mu} 6} \right) \\{{I\; 2^{2}} = {{\frac{1}{T}{\int_{c}^{a}{i\; 1(t)^{2}{t}}}} + {\frac{1}{T}{\int_{a}^{b}{i\; 3(t)^{2}{t}}}} + {\frac{1}{T}{\int_{a}^{b}{i\; 1(t)^{2}{t}}}} + {\frac{2}{T}{\int_{a}^{b}{\left\{ {i\; 3(t) \times i\; 1(t)} \right\} {t}}}} + {\frac{1}{T}{\int_{b}^{T}{i\; 1(t)^{2}{t}}}}}} & {\text{(}{formula}\mspace{14mu} 6\text{-}1\text{)}} \\{{I\; 2^{2}} = {{\frac{1}{T}{\int_{c}^{T}{i\; 1(t)^{2}{t}}}} + {\frac{1}{T}{\int_{a}^{b}{i\; 3(t)^{2}{t}}}} + {\frac{2}{T}{\int_{a}^{b}{\left\{ {i\; 3(t) \times i\; 1(t)} \right\} {t}}}}}} & \left( {{formula}\mspace{14mu} 6\text{-}2} \right)\end{matrix}$

In the formula 6, a square of c to a (the first term) is a square ofonly the instantaneous value i1(t) of the heater current I1. A sectionof a to be (the second term is a resultant current section of theinstantaneous value i3(t) of the power source current I3 and aninstantaneous value i1(t) of the heater current I1. Further, in theformula 6, a section of b to T (the third term) is a section of only theinstantaneous value i1(t) of the heater current I1. When the formula 6is developed, the formula 6-1 is obtained. When the formula 6-1 issummarized, the squared value of the inlet current I2 can be expressedby the formula 6-2. In the formula 6-2, the first term represents thesquared value of the heater current I1 in a section of c to T, and thesecond term represents the squared value of the power source current I3in a section of a to b. In the formula 6-2, the third term representsthe term generated by synthesizing the instantaneous value i3(t) of thepower source current I3 and the instantaneous value i1(t) of the heatercurrent I1 and then by squaring the resultant value. When the supplystarting timing c of the electric power supplied to the heater ischanged in a section from 0 to a, it is understood that the squaredvalue of the inlet current I2 is changed correspondingly to a change inthe squared value of the heater current I1 in the first term of theformula 6-2. In (c) of FIG. 12, the section from 0 to a corresponds to asection from 75% to 100% in fixing duty ratio in FIG. 11. That is, inthe case of (c) of FIG. 12, as also shown in FIG. 11, the slope of theinlet current I2 and the slope of the heater current I1 are equal toeach other.

Based on a principle described above, the slope of the squared value ofthe inlet current I2 is abrupt only in a region where the heater currentoverlaps with the power source current I3, and by using this principle,it is possible to detect the supply starting timing of the power sourcecurrent I3 and the supply ending timing of the power source current I3.In this embodiment, the supply starting timing of the power sourcecurrent I3 and the supply ending timing of the power source current I3are calculated from the slope of the squared value of the inlet currentI2. However, as the method of detecting the supply starting timing ofthe power source current I3 and the supply ending timing of the powersource current I3, in order to simplify the calculation or the like, amethod in which the timing is calculated from a waveform obtained bysubtracting the squared value of the heater current I1 from the squaredvalue of the inlet current I2. Further, as the method of detecting thesupply starting timing of the power source current I3 and the supplyending timing of the power source current I3, there is also a method inwhich the timing is calculated from a slope of a value obtained bysubtracting the heater current I1 from the inlet current value I2. Thus,these methods are not limited to those in this embodiment.

[Detecting Process of Supply Starting Timing and Supply Ending Timing ofPower Source Current]

A detecting process of the supply starting timing and the supply endingtiming of the power source current I3 passing through the power sourceunit 53 by using the inlet current detecting circuit 181 in thisembodiment will be described along a flowchart of FIG. 13 executed bythe CPU 32. In S301, the CPU 32 makes initial setting of respectivevariables. In this embodiment, the electric power supplied to the heater100 in an n-th half wave is Pn, and Pn=10 and n (counter)=0 are set. InS302, the CPU 32 actuates a motor or the like which operates during theimage formation. In S302, the CPU 32 discriminates whether or not thefalling edge of the zero-cross signal 502 outputted from the zero-crossgenerating circuit 57 is detected. In the case where the CPU 32discriminates that the falling edge of the zero-cross signal 502 is notdetected, the CPU 32 repeats the process of S303. In S303, in the casewhere the CPU 32 discriminates that the falling edge of the zero-crosssignal 501 is detected, in S304, the CPU 32 increments the suppliedelectric power Pn and the counter n by 1 in S304 (Pn=Pn+1, n=n+1).

In S305, the CPU 32 supplies the electric power of Pn % to the heater100.

In S306, the CPU 32 discriminates whether or not the rising edge of thezero-cross signal 502 outputted from the zero-cross generating circuit57, and in the case where the CPU 32 discriminates that the rising edgeof the zero-cross signal 502 is not detected, the CPU 32 repeats theprocess of S306. In S306, in the case where the CPU 32 discriminatesthat the rising edge of the zero-cross signal 502 is detected, thesequence goes to a process in S307. In S307, the CPU 32 detects theinlet current, when the electric power of Pn % is supplied in S305, bythe inlet current detecting circuit 181, and then obtains a squaredvalue of the inlet current. Hereinafter, the inlet current when theelectric power of Pn % is supplied will be described as I2n. The squarevalue thereof is I2n².

In S308, the CPU 32 discriminates whether or not the counter n is 2 ormore, and in the case where the CPU 32 discriminate, that the counter nis not 2 or more, the sequence returns to the process of S303. In S308,in the case where the CPU 32 discriminates that the counter n is 2 ormore, the sequence goes to a process of S309. In S309, the CPU 32applies a change amount An of the slope of the squared value of theinlet current I2n to the supplied electric power Pn %.

An = {I 2n² − I 2(n − 1)²}/{Pn − P(n − 1)} − {I 2(n − 1)² − I 2(n − 2)²}/{P(n − 1) − P(n − 2)}

The calculated value of An is stored in, e.g., RAM 32 b.

In S310, the CPU 32 discriminates whether or not the supplied electricpower Pn % is 90 or more, and in the case where the CPU 32 discriminatesthat the supplied electric power Pn % is not 90 or more, the sequencereturns to the process of S303. That is, the CPU 32 calculates the inletcurrent I2n at the supplied electric power Pn % and the change amount Anof the slope of the squared value of the inlet current I2n to each ofvalues of the supplied electric power Pn % in the processes S303 to S309until the supplied electric power Pn % reaches 90 or more. In S310, inthe case where the CPU 32 discriminates that the supplied electric powerPn % is 90 or more, the sequence goes to a process of S311.

In S311, the CPU 32 reads out the value of An stored in the RAM 32 b,and obtains a duty ratio PA max when the change amount An of the slopeof the squared value of the inlet current I2n to the supplied electricpower Pn % is maximum, and a duty ratio PA mm when the change amount Anis minimum. The duty ratios PAmax and PAmin are not limited to thoseobtained in this embodiment, but may also be those obtained by otherknown extracting methods of maximum and minimum values. As describedwith reference to FIG. 11, the point where the slope changes from amoderate slope to an abrupt slope is the supply ending timing of thepower source current I3, and the point where the slope changed from theabrupt slope to the moderate slope is the supply starting timing of thepower source current I3. At the point changed in slope from the moderateslope to the abrupt slope, a slope change amount A is maximum. At thepoint changed in slope from the abrupt slope to the moderate slope, theslope change amount A is minimum. Accordingly, PAmax obtained in S311 isthe supply ending timing of the power source current I3, and PAmin isthe supply starting timing of the power source current I3. The values ofPAmax and PAmin calculated by the CPU 32 in S311 are stored in, e.g.,the RAM 32 b.

In the case where the detecting process in this embodiment is applied toEmbodiments 1 and 2, e.g., detecting process in this embodiment iscarried out during the process of S102 in FIG. 7 or 10. The detectingprocess in this embodiment may also be carried out before the process ofS102 in FIG. 7 or 10.

As described above, by the detecting process in this embodiment, it ispossible to detect the supply starting timing of the power sourcecurrent I3 and the supply ending timing of the power source current I3.In this embodiment, the duty ratio is gradually changed from 10% to 90%in an increment of 1% (S304 and S310 in FIG. 13), so that the supplystarting timing and the supply ending timing of the power source currentI are obtained. However, when the duty ratio is changed, in an initialstage, the duty ratio is changed in a large increment, so that thesupply starting timing and the supply ending timing of the power sourcecurrent I may also be roughly calculated. In this case, in theneighborhood of each calculated timing, the duty ratio is changed in asmall increment, so that the supply starting timing and the supplyending timing of the power source current I3 are detected. Further, thedetecting process in this embodiment may also be carried out only in theneighborhood of each of the supply starting timing and the supply endingtiming of the power source current I3. In this case, the time requiredfor the timing detecting process can be shortened. In this way, thedetecting method of the supply starting timing and the supply endingtiming of the power source current I3, and the number and the order ofmeasurements of the inlet current I2n are not limited to those in thisembodiment.

As described above, according to this embodiment, it is possible toimprove the power factor while realizing the downsizing of the imageforming apparatus and the cost reduction.

Embodiment 4

In Embodiment 3, the example in which the duty ratio of the electricpower supplied to the heater is changed from 10% to 90% in the incrementof 1% to calculate the supply starting timing of the power sourcecurrent I3 and the supply ending timing of the power source current I3was described. In Embodiment 4, a constitution in which whether or notthe supply starting timing of the power source current I3 and the supplyending timing of the power source current I3 are fluctuated depending ona variation in load or the commercial power source voltage 501 ischecked is employed. Further, an example in which the supply startingtiming and the supply ending timing of the power source current I3 aredetected again and then whether or not there is a need to make thechange is discriminated will be described. Constitutions similar tothose described in Embodiments 1 to 3 will be omitted from descriptionby adding the same reference numerals or symbols.

[Discriminating Process Whether or not Change is Needed (Start Timing)]

FIG. 14 is a flowchart for illustrating a control process of adiscriminating sequence, by the CPU 32, as to whether or not supplystarting timing of the power source current I3 should be changed in thisembodiment. In FIG. 14, the steps as those in FIG. 13 will be omittedfrom description by adding the same step symbols. Further, the inletcurrent in this embodiment will be described by adding a symbol of I1k(and I2k² as a squared value) in order to discriminate the inlet currentI1k from the inlet current I2n in Embodiment 3.

In S401, the CPU 32 makes initial setting of respective variables. Thatis, the CPU 32 sets supplied electric power Pk at PAmin−1 (Pk=PAmin−1),and sets a counter k at 0 (k=0). That is, as the supplied electric powerPk, a value which is smaller than the supply starting timing PAmin ofthe power source current I3 by 1% is set. The process of S303 is thesame as that described in Embodiment 3, and therefore will be omittedfrom description. In S402, the CPU 32 supplies the electric power Pk %.The process of S306 is the same as that described in Embodiment 3, andtherefore will be omitted from description.

In S403, the CPU 32 detects the inlet current, when the electric powerof Pk % is supplied in S402, by the inlet current I1k detecting circuit181, and then obtains a squared value of the inlet current I2k. In S404,the CPU 32 increments the supplied electric power Pk % and the counter kby 1 (Pk=Pk+1, k=k+1). In S405, the CPU 32 discriminates whether or notthe counter k is 2 or more, and in the case where the CPU 32discriminate, that the counter k is not 2 or more, the sequence returnsto the process of S303.

In S405, in the case where the CPU 32 discriminates that the counter nis 2 or more, in S406, the CPU 32 applies a change amount Ak of theslope of the squared value of the inlet current I2k to the suppliedelectric power Pk %.

An = {I 2k² − I 2(k − 1)²}/{Pk − P(k − 1)} − {I 2(k − 1)² − I 2(k − 2)²}/{P(k − 1) − P(k − 2)}

In S407, the CPU 32 discriminates whether or not an absolute value |Ak|of the slope change amount Ak is larger than a predetermined slopechange amount Ax. The predetermined slope change amount Ax is athreshold for discriminating whether or not the slope change amount Akchanges. In S407, in the case where the CPU 32 discriminates that theabsolute value |Ak| of the slope change amount Ak is larger than thepredetermined slope change amount Ax, the CPU 32 discriminates that theslope change amount Ak of the squared value of the inlet current I2k,and then the sequence goes to a process of S408. In S401, the CPU 32sets, as the supplied electric power Pk %, a value which is 1% smallerthan the supplied electric power PAmin corresponding to that at thesupply starting timing of the power source current I3. For this reason,in S406, the CPU 32 calculates the slope change amount in theneighborhood of the supplied electric power PAmin %, and when the slopeof the squared value of the inlet current I1k changes, |Ak| is largerthan the predetermined slope change amount Ax. In S408, the CPU 32discriminates that there is no need to make the change since the supplystarting timing of the power source current I3 is not fluctuated.

On the other hand, in S407, in the case where the CPU 32 discriminatesthat the absolute value |Ak| of the slope change amount Ak is not largerthan the predetermined slope change amount Ax, i.e., in the case wherethe slope of the squared value of the inlet current I1k does not change,the sequence goes to a process of S409. In S409, the CPU 32 detects thatthe change is needed since the supply starting timing of the powersource current I3 fluctuates. In S409, in the case where the CPU 32discriminates that the change in supply starting timing of the powersource current I3 is needed, the CPU 32 carries out the detectingprocess of the supply starting timing of the power source current I3described in Embodiment 3.

[Discriminating Process Whether or not Change is Needed (End Timing)]

A discriminating process as to whether or not supply ending timing ofthe power source current I3 should be changed in this embodiment will bedescribed with reference to FIG. 15. In FIG. 15, the steps in which thesame processes as those in the flow chart described with reference toFIG. 14 will be omitted from description by adding the same stepsymbols. In S411, the CPU 32 makes initial setting of respectivevariables. That is, the CPU 32 sets supplied electric power Pk atPAmax−1 (Pk=PAmax−1), and sets a counter k at 0 (k=0). That is, as thesupplied electric power Pk, a value which is smaller than the supplyending timing PAmax of the power source current I3 by 1% is set. Theprocesses of S303 to S406 are the same as those described with referenceto FIG. 14, and therefore will be omitted from description.

In S407, in the case where the CPU 32 discriminates that the absolutevalue |Ak| of the slope change amount Ak, calculated in S406, of thesquared value of the inlet current I2 is larger than the predeterminedslope change amount Ax, the sequence goes to a process of S412. In S412,the CPU 32 discriminates that there is no need to make the change sincethe supply ending timing of the power source current I3 is notfluctuated. On the other hand, in S407, in the case where the CPU 32discriminates that the absolute value |Ak| of the slope change amount Akof the squared value of the inlet current I2k is the predetermined slopechange amount Ax or less, the sequence goes to a process of S413. InS413, the CPU 32 detects that the change is needed since the supplyending timing of the power source current I3 fluctuates. In S413, in thecase where the CPU 32 discriminates that the change in supply endingtiming of the power source current I3 is needed, the CPU 32 carries outthe detecting process of the supply ending timing of the power sourcecurrent I3 described in Embodiment 3.

In this way, the CPU 32 carries out the detecting process in Embodiment4 only in the case where the CPU 32 discriminates that the supplystarting timing or the supply ending timing of the power source currentI3 fluctuates. Then, the CPU 32 updates information of the supplystarting timing or the supply ending timing of the power source currentI3 stored in the RAM 32 b. Then CPU 32 carries out the electric powercontrol process for improving the power factors in Embodiments 1 and 2by using PAmax or PAmin which is not updated (not fluctuated) or isupdated (is fluctuated).

As described above with reference to FIGS. 14 and 15, even in the casewhere the supply starting timing and the supply ending timing of thepower source current I3 change depending on fluctuations in power sourcevoltage and load, it is possible to detect the change in a short time bythe process of this embodiment. As described above, according to thisembodiment, it is possible to improve the power factor while realizingthe downsizing of the image forming apparatus and the cost reduction.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth and thisapplication is intended to cover such modifications or changes as maycome within the purpose of the improvements or the scope of thefollowing claims.

This application claims priority from Japanese Patent Application No.207243/2013 filed Oct. 2, 2013, which is hereby incorporated byreference.

What is claimed is:
 1. An image forming apparatus comprising: a powersource portion for converting an AC voltage of a commercial power sourceinto a DC voltage; a fixing portion for heating and fixing an image,formed on a recording material, on the recording material, wherein saidfixing portion includes a first heat generating element which generatesheat by electric power supplied from the commercial power source and asecond heat generating element which is controlled independently of thefirst heat generating element and which generates the heat by theelectric power supplied from the commercial power source; and acontroller for controlling the electric power supplied to the first andsecond heat generating elements, wherein when a plurality of periods ofan AC waveform of the commercial power source constitute one controlperiod, said controller sets a waveform of a current to be passedthrough each of the first and second heat generating elements in the onecontrol period so that total electric power supplied to the first andsecond heat generating elements in the one control period is dependenton a temperature of said fixing portion, wherein said controller setsthe waveform of the current to be passed through each of the first andsecond heat generating elements so that, in an equiphase half wave in atleast a part of the one control period, the current passes through oneof the first and second heat generating elements from a halfway point ofthe half wave and the current passes through or does not pass throughthe other heat generating element throughout a period of the half wave,and wherein said controller sets a current supply starting timing of thecurrent passing through the one of the first and second heat generatingelements from the halfway point of the half wave, at timing when acurrent passing toward said power source portion stops.
 2. An imageforming apparatus according to claim 1, further comprising a currentdetecting portion for detecting a resultant current passing through saidpower source portion and the first and second heat generating elements,wherein said controller obtains timing, when the supply of the currenttoward said power source portion, on the basis of a duty ratio of theelectric power supplied to the one of the first and second heatgenerating elements through which the current passes through from thehalfway point of the half wave and a detection result of said currentdetecting portion.
 3. An image forming apparatus according to claim 1,wherein said fixing portion includes a cylindrical fixing film.
 4. Animage forming apparatus according to claim 3, wherein the first andsecond heat generating elements are provided on a single heaterincluding a ceramic substrate, and wherein the heater contacts an innersurface of the fixing film.
 5. An image forming apparatus comprising: afixing portion for heating and fixing an image, formed on a recordingmaterial, on the recording material, wherein said fixing portionincludes a first heat generating element which generates heat byelectric power supplied from the commercial power source and a secondheat generating element which is controlled independently of the firstheat generating element and which generates the heat by the electricpower supplied from the commercial power source; and a controller forcontrolling the electric power supplied to the first and second heatgenerating elements; wherein said controller switches a rule of awaveform of an AC current to be passed through each of the first andsecond heat generating elements depending on a duty ratio of totalelectric power supplied to the first and second heat generatingelements.
 6. An image forming apparatus according to claim 5, furthercomprising a power source portion for converting an AC voltage of acommercial power source into a DC voltage, wherein each of the first andsecond heat generating elements is supplied with the electric power fromthe commercial power source without via said power source portion.
 7. Animage forming apparatus according to claim 6, wherein when a pluralityof periods of an AC waveform of the commercial power source constituteone control period, said controller sets the duty ratio of the totalelectric power depending on a temperature of said fixing portion everyone control period.
 8. An image forming apparatus according to claim 7,wherein said controller switches a first rule in which in an equiphasehalf wave over the one control period, a current is passed through oneof the first and second heat generating elements from a halfway point ofthe half wave and the current is passed through or is not passed throughthe other heat generating element throughout a period of the half wave,and a second rule in which in the equiphase half wave in at least a partof the one control period, the current is passed through both of thefirst and second heat generating elements throughout the period of thehalf wave.
 9. An image forming apparatus according to claim 8, whereinthe duty ratio of the total electric power in which the second rule isset is larger than the duty ratio of the total electric power in whichthe first rule is set.
 10. An image forming apparatus according to claim5, wherein a resistance value of the first heat generating element and aresistance value of the second heat generating element are differentfrom each other.
 11. An image forming apparatus according to claim 5,wherein said fixing portion includes a cylindrical fixing film.
 12. Animage forming apparatus according to claim 11, wherein the first andsecond heat generating elements are provided on a single heaterincluding a ceramic substrate, and wherein the heater contacts an innersurface of the fixing film.
 13. An image forming apparatus comprising: apower source portion for converting an AC voltage of a commercial powersource into a DC voltage; a fixing portion for heating and fixing animage, formed on a recording material, on the recording material,wherein said fixing portion includes a first heat generating elementwhich generates heat by electric power supplied from the commercialpower source and a second heat generating element which is controlledindependently of the first heat generating element and which generatesthe heat by the electric power supplied from the commercial powersource; a current detecting portion for detecting a resultant currentpassing through said power source portion and the first and second heatgenerating elements; and a controller for controlling the electric powersupplied to the first and second heat generating elements, wherein saidcontroller sets a length of the one control period depending on adetected current of said current detecting portion.
 14. An image formingapparatus according to claim 13, wherein when the detected current islarger than a predetermined current, said controller sets the length ofthe one control period is set so as to be longer than that when thedetected current is smaller than the predetermined current.
 15. An imageforming apparatus according to claim 13, wherein said fixing portionincludes a cylindrical fixing film.
 16. An image forming apparatusaccording to claim 13, wherein the first and second heat generatingelements are provided on a single heater including a ceramic substrate,and wherein the heater contacts an inner surface of the fixing film.